The present invention is related to the field of radiation emitting devices. More in particular semiconductor devices that emit light at a predetermined wavelength with a high efficiency are disclosed. A method of making such devices and applications of the devices are also disclosed.
Semiconductor devices that can emit non-coherent or coherent light are known in the art. A number of publications on semiconductor based light emitters deals with Light Emitting Diodes (LEDs) or Microcavity LEDs or Microcavity Lasers or Vertical Cavity Surface Emitting Lasers. Examples of such publications are:
H. De Neve, J. Blondelle, R. Baets, P. Demeester, P. Van Daele, G. Borghs, IEEE Photon. Technol. Lett. 7 287 (1995);
E. F. Schubert, N. E. J. Hunt, R. J. Malik, M. Micovic, D. L. Miller, xe2x80x9cTemperature and Modulation Characteristics of Resonant-Cavity Light-Emitting Diodesxe2x80x9d, Journal of Lightwave Technology, 14 (7), 1721-1729 (1996);
T. Yamauchi and Y. Arakawa, Enhanced and inhibited spontaneous emission in GaAs/AlGaAs vertical microcavity lasers with two kinds of quantum wells. Appl. Phys. Lett. 58 (21), 2339 (1991);
T. J. de Lyon, J. M. Woodall, D. T. McInturff, R. J. S. Bates, J. A. Kash, P. D. Kirchner, and F. Cardone, xe2x80x9cDoping concentration dependence of radiance and optical modulation bandwidth in carbon-doped Ga0.1 In0.49P/GaAs light-emitting diodes grown by gas source molecular beam epitaxyxe2x80x9d Appl. Phys. Lett. 60 (3), 353-355 (1992);
D. G. Deppe, J. C. Campbell, R. Kuchibhotla, T. J. Rogers, B. G. Streetman, xe2x80x9cOptically-coupled mirror-quantum well InGaAs-GaAs light emitting diodexe2x80x9d, Electron. Lett. 26 (20), 1665 (1990);
M. Ettenberg, M. G. Harvey, D. R. Patterson, xe2x80x9cLinear, High-Speed, High-Power Strained Quantum-Well LED""sxe2x80x9d, IEEE Photon. Technol. Lett. 4 (1), 27 (1992);
U.S. Pat. No. 5,089,860 Deppe, et. al. Feb. 18, 1992, xe2x80x9cQuantum well device with control of spontaneous photon emission, and method of manufacturing samexe2x80x9d.
It is known in the art that the light emission from an electroluminescent device or from a light emitting semiconductor diode (a LED) is limited by the total interal reflection occurring at the interface between the semiconductor substrate wherein the device is fabricated and the surrounding medium. Mostly emission of the light to air, with refractive index of unity, is intended. The semiconductor typically has a refractive index ns of 3 to 4. GaAs, for example, has a refractive index ns=3.65. Snell""s law determines that only photons arriving at the semiconductor-air interface with an angle smaller than a critical angle xcex8c=arcsin(1/ns) can escape to the air. All other photons are totally reflected at the semiconductor-air interface, and therefore remain in the semiconductor substrate, until eventually they are re-absorbed. For GaAs, the critical angle for total internal reflection is 16 degrees. Hence, total internal reflection limits the number of photons escaping the semiconductor substrate to those photons arriving at the semiconductor-air interface with an angle of less than 16 degrees. Only about 2% of the photons generated inside the semiconductor comply with this condition.
Several prior-art inventions propose to increase the escape probability of photons generated in the LED. In microcavity light-emitting diodes, such as described for example by Cho et al. in U.S. Pat. No 5,226,053, the active layer of the light emitting device is placed in a microcavity. The cavity influences the emission of the photons: more photons are generated with an angle smaller than the critical angle xcex8c. In this way, efficiencies of 15% and more have been achieved.
A second way to increase the efficiency of LEDs is to re-absorb photons which cannot escape from the semiconductor. If re-absorption occurs in the active layer of the LED, there is a chance that the electron-hole pair generated during re-absorption will recombine radiatively again, and re-emit a photon. Again, 2% of these photons will escape, and the remainder part can be re-absorbed. The phenomenon of multiple re-absorption and re-emission has been shown to result in efficiencies of the order of 10% in normal LEDs, and also to boost the efficiency of certain microcavity LEDs up to 23%. The problem with this technique is that it is inherently slow, because one has to wait for multiple re-absorptions and re-emissions.
A third way is to shape the semiconductor surface of the light emitting devices such that more of the generated rays reach the semiconductor-air interface within the critical angle. The optimum shape for the semiconductor-air surface is a hemisphere, where the light-emitting area is confined to a small spot at the centre of the hypothetical full sphere from which the hemisphere is taken. Other shapes have been proposed. In U.S. Pat. No. 5,087,949, Haitz proposes a structure which is more practical to make than a hemisphere, namely a set of V-groves in the substrate that are created such that the normal to the V-grove facets are oriented substantially perpendicularly to the light-emitting region. Kato, in U.S. Pat. No. 5,349,211, proposes a structure where the sidewalls or edges of the substrate are shaped such that some of the photons that are reflected from the regular light-output interface are emitted through these sidewalls. Egalon and Rogowski propose a sidewall shape for the substrate (rather than only for the mesa) that redirects some of the photons to angles that can escape through the regular light-output surface. All these proposed structures assume that the LED substrate is fairly thick and transparent for the photons emitted by the diode.
According to the teaching of U.S. Pat. No. 5,087,949 by Yamanaka et al. light emitting devices with a cavity having a truncated polyhedral pyramid shape are created. The lateral edges or facets of the devices have an angle of preferentially 45 degrees. Photons that are generated in a direction parallel to the light-output surface (edge) are reflected by the mesa edge of such cavity into a direction which is substantially perpendicular to the light-output surface, and hence more photons can escape from the cavity.
A fourth way to increase LED efficiency is to provide device structures capable to redirect a photon more than once before the photon is re-absorbed. This goal is achieved by providing surface edges of the device that contain surface portions having angles different from the main semiconductorsubstrate-air interface. Every time a photon hits such a surface, it is redirected to a new propagation angle. In this way, photons travelling in a direction that is not favourable for emission to the air have a certain probability to be redirected in a favourable angle after a number of reflections at such surface. In U.S. Pat. No. 3,739,217, Bergh and Saul propose to create topographic irregularities at the light-emitting surface or at the opposite (light-reflecting) surface of a LED which has a transparent substrate. Noguchi et al. in EP-A-0404565 propose to texture the sidewalls or edges of the substrate wherein the light emitting device is made.
The previously described methods apply to light-emitting devices where the light is emitted through the substrate. Therefore, the light-emitting devices need to be fabricated in a transparent substrate. The teaching of the patents of Noguchi et al., Egalon et al., and Kato et al. can only be applied to single light-emitting devices, but not to light-emitting devices in arrays. The invention disclosed by Haitz et al. (U.S. Pat. No. 5,087,949) can be applied to arrays, but it requires the presence of a fairly thick substrate, and the spacing of the light-emitting devices of an array should be of the order of the substrate thickness.
A method for manufacturing light-emitting devices the substrate of which does not have to be transparent is proposed in U.S. Pat. No. 5,358,880, by Lebby et al. The invention includes replacing the original substrate, which can be non-transparent, by a transparent conductive layer such as Indium Tin Oxide and a clear epoxy plus glass-like host substrate. Further, the disclosed invention aims to make a closed cavity, by etching through the active layer of the light emitting device, and covering the sides of the etched mesa with a dielectric material and a metal. In this way, photons that cannot escape through the transparent window are kept in the cavity by multiple reflections, re-absorbed and eventually re-emitted.
When applied to for instance a material system like GaAs, InAs, or AlAs and combinations thereof, the inventions mentioned above can only be used for light-emitting devices with a large diameter. This is because these prior art inventions rely on etching through the active layer of the light-emitting device. When doing so in a material with a large surface recombination velocity like GaAs, one triggers a severe parasitic surface recombination current. Surface recombination is a phenomenon of two charge carriers of opposite type, an electron and a hole, recombining at a trap, e.g. at the surface of the material, without emitting light. The velocity of recombination is proportional to the number of available traps and to the density of charge carriers available for recombination. The surface of III-V semiconductors like GaAs has a large number of traps, about 1014 cmxe2x88x922. Hence, the surface recombination velocity is very large. Thus there is a problem of making highly efficient light emitting devices as the recombination losses of the charge carriers decrease the efficiency of the total light emission of the devices. One solution is to inject electrons and holes from electrodes that are physically at a large distance from the free surface (edge), which usually coincides with the cleavage surface of the device. Because of the necessity of having a large distance between the edge of the device and the contact, the resulting device is necessarily large, typically at least several hundred microns in diameter. Such devices therefore are not suitable for integration in large arrays. Also, their large area results in a large capacitance, and therefore slow operation.
Thus the prior art fails to disclose highly efficient light-emitting devices that can be integrated as small devices in a dense array of light emitting devices.
High efficiency LEDs are required for applications such as optical communication. Optical communication replaces electrical communication in many areas, because it can provide longer distance interconnects for a given energy budget. The minimum distance over which optical interconnects are advantageous over electrical interconnects can be quite short, e.g. a few centimetres. The energy consumption necessary for a given data bandwidth is a critical factor deciding on the minimum distance over which optical interconnects are competitive over electrical interconnects. The bandwidth transmitted by an optical interconnect system is the product of the serial bandwidth per interconnect channel with the number of parallel optical channels. Optics have the advantage over electrical interconnects that the number of channels can be much larger. One of the conditions to be able to access this potential massive parallelism, is that the power consumption per channel remains small: the heat dissipation has to remain manageable. Therefore high efficiency LEDs that can be integrated in dense arrays are needed.
The present invention aims to disclose radiation, preferably light, emitting devices with a high radiation emission efficiency. The invention further aims at disclosing radiation, preferably light, emitting devices that can be fabricated as small devices in an array of such devices.
According to an aim of the invention the radiation, preferably light, emitting devices can be placed in dense arrays.
According to another aim of the invention the outcoupling efficiency of radiation, preferably light, emitting devices is improved, which leads to a reduced power consumption for a given radiation output power.
According to yet another aim of the invention, the speed of the radiation, preferably light, emitting devices is increased, hence the serial bandwidth per optical channel is increased. The invention further aims to disclose light emitting devices that exhibit uniform radiation emission characteristics.
According to yet another aim of the invention, light-emitting devices are disclosed having a low-resistance contact path. In this way a high wall-plug efficiency is achieved. Hereto in inventive embodiments of the invention, electrical contacts are foreseen through at least one hole in a mirror side of the light-emitting device. In such embodiment, the mirror preferably is not conductive.
The light emitting devices that are disclosed can be used in parallel optical interconnects. The light emitting devices according to the invention can be placed in dense arrays. The light outcoupling efficiency is strongly improved, which leads to a reduced power consumption for a given optical output power. Third, the speed of the devices is increased, hence the serial bandwidth per optical channel is increased.
The features of the devices (diodes) of the present invention can be applied for a multitude of light emitting devices such as high-efficiency LEDs or microcavity LEDs.
The light emitting devices (diodes) of the present invention can be used for applications wherein two-dimensional LED arrays, particularly low-power arrays, are useful, such as in display technology. Active matrix displays relying on liquid crystals (e.g. integrated on CMOS circuitry) could be replaced by LED arrays. Dense and bright one-dimensional LED arrays are useful for example for printing and copying applications.
Also for single LED applications it is important to have a maximum of photons escaping from the light emitting surface. Firstly, the intensity of light per unit area (the brightness) is larger, and this is useful in many applications. Furthermore, the packaging cost can be reduced. Indeed, in order to achieve a large global efficiency, many conventional LEDs need an elaborate package that includes a cavity with mirrors, because the light is emitted from more than one surface of the LED.
In an object of the present invention radiation, preferably light, emitting devices and methods of making such devices are disclosed that have a high radiation emission efficiency. According to this aspect of the invention the outcoupling efficiency of radiation, preferably light, emitting devices is improved, which leads to a reduced power consumption of the devices for a given radiation output power.
In another object of the present invention radiation, preferably light, emitting devices and methods of making such devices are disclosed that can be fabricated as small devices in an array of such devices.
The radiation, preferably light, emitting devices of the present invention can have an increased speed performance, hence the serial bandwidth per optical channel is increased. The radiation, preferably light, emitting devices of the present invention can exhibit uniform radiation emission characteristics.
For the purpose of this patent application a number of terms are defined herebelow. A cavity of a radiation emitting device is the space within the device containing the active layer of the device and being enclosed by edges, at least one of the edges having reflective characteristics. The space furthermore has at least one window or transparent edge such that photons with a predetermined wavelength or within a predetermined wavelength band can escape through this window. The active layer of the radiation-emitting device is the region of the device wherein the charge carriers of the device, for instance electrons and holes, meet for creating radiation (light). Mesa edges are the edges of radiation emitting devices that are defined by etching structure in or growing structure upon a substrate. Typically a device with mesa edges has a table-like or truncated-pyramid-like shape. The term edge in this patent application is to be understood as a sidewall or a surface or a delimiting surface of the devices of the invention. A predetermined wavelength of course is to be understood as including a limited wavelength band around said predetermined wavelength. With the term confined through form and functioning it is meant that without additional means or without additional structural features such as extended, large isolation features in an array of devices, each of the devices in an array of devices can be addressed individually via the connection for electrical signals. Such confinement through form and functioning can be done e.g. when said devices are being integrated in a single thin film semiconductor.
In a first aspect of the present invention, a device for emitting radiation at a predetermined wavelength is disclosed. Said device is mounted on a carrier substrate, said carrier substrate being transparent for said radiation and preferably including a fiber optic face plate. This allows for denser arrays, because otherwise the pitch of an array of devices is limited by the glass thickness. The device can have the edge being transparent for said radiation having a roughened reflective surface condition. Also an edge being transmissive for said radiation can have a roughened surface condition. In a preferred embodiment of the invention, the roughened surface condition is present as a substantially random diffraction grating structure.
In a second aspect of the present invention, a device for emitting radiation at a predetermined wavelength is disclosed, said device having a cavity with an active layer wherein said radiation is generated by charge carrier recombination, said cavity comprising at least one edge having a substantially random grating structure. The edges of the device define the region or space for radiation and/or charge carrier confinement. Said edge having a substantially random grating structure can extend as at least one edge of a waveguide forming part of said device. The radiation emitting device of the present invention can have a cavity comprising a radiation confinement space that includes confinement features for said charge carriers confining said charge carriers to a subspace being smaller than the radiation confinement space within said cavity. The waveguide forming part of the device can according to such embodiment of the invention be the region of radiation confinement of the device that is larger than the electrical confinement region. The edge of the device extending as a waveguide forming part of said device and having a substantially random partially reflective grating structure can also abut said active layer or extend in said active layer. Thus a way to avoid photons escaping by guided modes from the mesa area is disclosed. One of the edges of the cavity of the device can be a mesa edge. According to another preferred embodiment of the invention the region where charge carriers meet for creating radiation (the active layer) has none or substantially none free surface or edge where surface recombination can take place. Thus the radiation emitting device of the present invention can have a cavity comprising a radiation confinement space that includes confinement features for said charge carriers confining said charge carriers to a subspace being smaller than the radiation confinement space within said cavity. The device according to this aspect of the invention can have a cavity comprising at least one mesa edge for defining said radiation confinement space and said device can further comprise a ring of a dielectric material in said cavity. Thus according to this preferred embodiment of the present invention, the devices are optically and electrically confined and the electrical confinement space is a subspace of the optical confinement space. The devices according to this embodiment of the present invention can be confined in form and functioning without additional means or without additional structural features such as extended, large isolation features in the device. The device can operate while strongly reducing the recombination effects of charge carriers in said device. Such confinement in form and functioning can be done e.g. when said devices are being integrated in a single thin film semiconductor. In preferred embodiments of the invention, the confinement in form and functioning is done by a double mesa edge. The electrical confinement of the devices of the invention to a space that is a subspace of the optical confinement space can be achieved by the presence of a ring of dielectric material in the cavity of the device. The double mesa edge can comprise a first mesa edge above a second mesa edge wherein the active layer of said device is located. The device can also have an active layer that is located above a ring of a dielectric material in said cavity. Also double mesas are disclosed that include an isolation plus metal coating on the mesa edges. Such is important for reducing the surface recombination in small radiation emitting devices. A combination of single mesas with oxidised aperture is also disclosed. The surfaces or edges of the cavity can be textured or roughened. This texturing can be smaller or larger than the wavelength of the radiation and can be a substantially random grating structure pattern. The texturing or roughening adds to more scattering of photons inside the cavity, and therefore again improves the efficiency and the speed of the devices.
The devices of the present invention do not rely on a resonance for their operation, and their performance is not critical impacted by variations in growth and process. They are therefore suitable for integration into large arrays. Furthermore, they exhibit high efficiency characteristics, in particular at high speed.
In a third aspect of the present invention, an array of devices is disclosed, wherein individual devices of said array are being confined in form and functioning. At least one of the edges of the devices of the array can have a substantially random grating structure and at least one edge can extend as a waveguide forming part of said device. The radiation emitting devices of the array can have a cavity comprising a radiation confinement space that includes confinement features for said charge carriers confining said charge carriers to a subspace being smaller than the radiation confinement space within said cavity. The grooves inbetween the individual devices of the array can have a substantially random diffraction grating structure. The array of devices can have a single electrical anode contact and a single electrical cathode contact. The devices of the array can be thinned and be mounted on a carrier substrate. A microlense or microlense array can be positioned on the array of light emiting devices to enhance the light output efficiency and optimize the beam profile.
In a fourth aspect of the present invention, a radiation emitting device is disclosed wherein said device has a cavity wherein said radiation is generated, and said device can comprise at least two adjacent or abutting edges of said cavity forming in cross-section a substantially triangular shape and the angle between said edges being smaller than 45 degrees and at least one of said edges having a transparent portion. In an embodiment the two edges can be adjacent or abutting one to another and one of the edges is transparent and the other is reflective. The non-transparent edge can be a mesa edge. Also the transparent edge can be a mesa edge. Thus the edges or one of the edges can extend essentially throughout the whole thickness of the device, the thickness of the device being measured between the mesa edge and the transparent window edge. The edges can have a roughened surface condition. In a preferred embodiment of this aspect of the invention, the mesa edge is tapered with angles xe2x89xa745 degrees. In this way, not only photons having one specific direction get out. Also, the photons get out after fewer passes, and less re-combination and re-emission occurs. Therefore, a higher speed performance of the device is achieved.
In a fifth aspect of the present invention, a thin-film light emitting device has a reflective edge such as a metal mirror, preferably dielectric-coated, and an electrical contact is provided through this reflective edge or mirror. The carrier substrate can be located at the mirror side of the thin-film semiconductor device, and light can escape through the opposite side. The carrier substrate can be electrically conductive. It can also be a heat-sink for the LED. Thus according to this fifth aspect of the present invention, light-emitting devices are disclosed having a low-resistance contact path. In this way a high wall-plug efficiency is achieved. Hereto, electrical contacts are foreseen through at least one hole in a mirror side of the light-emitting device. In such embodiment, the mirror preferably is not conductive.
The features of the above-described aspects of the invention can be combined in any way to achieve a very efficient device for emitting radiation at a predetermined wavelength.
In a sixth aspect of the present invention, a method for texturing at least a part of at least one surface of a substrate is disclosed. The method comprises the steps of applying preferably substantially randomly an overlayer material covering part of said surface, said overlayer material having a pattern with substantially random distributed open features; and etching said surface while using said overlayer material as an etching mask, said mask thereby containing a substantially random masking pattern. The pattern of the overlayer material with substantially random distributed open features can have any configuration or shape. It can comprise holes therein said holes being randomly distributed. The holes can also partly overlap one another. The substantially random masking pattern can comprise regular repetitions of random subpatterns.
The step of applying said overlayer material with substantially random distributed open features can include the substeps of applying a layer of photoresist material and illuminating said photoresist material with a lithography mask having a substantially random masking pattern and thereafter developing said photoresist. The developed photoresist forms then an overlayer material with substantially random distributed features. UV, deep-UV, x-ray, electron-beam or any type of lithography can be used to illuminate the photoresist. The mask can be a metal mask for a contact aligner or a stepper. It can also be generated as a specular noise pattern of a coherent light source such as a laser.
The method can also comprise the steps of applying a substantially random distribution of particles on said surface; reducing the size of said particles; and thereafter etching said surface while using said particles as an etching mask. Thus it means that the etching techniques that are used are not degrading the particles or that the particles are such that the particles are resistant against the etchants used or have undergone a treatment for making the particles resistant against the etchants used.
The etching step can be to roughen said surface, said surface thereby being rendered diffusive for electromagnetic radiation impinging on said surface. The etching step can comprises the step of etching pillars in said substrate. In a preferred embodiment of the invention, said particles are applied in a monolayer 2-dimensional partial coverage of said surface. The applied partial coverage can be about 60% of the surface and after the size reduction of the particles yielding a coverage of about 50% of the surface.
The method can further comprise the steps of applying a layer of a second masking material on said surface; developing a pattern in said second masking material while reducing the size of said particles; and thereafter etching said surface while using said pattern as an etching mask.
In a seventh aspect of the invention, the method can further comprise the steps of preparing in said substrate a device for emitting radiation at a predetermined wavelength, said device having a cavity, said surface being at least a part of one of the edges of the cavity of the device. According to this aspect of the invention, the reduced size of said particles can be in a range of 50% to 200% of the wavelength of said radiation in the substrate. The edge of the device having a roughened surface condition can be a transmissive and can be a reflective edge of the device. The devices of the first, second and third aspect of the invention can have edges that are roughened according to this method. The roughened edges have then a substantially random diffraction grating structure.
Any of the embodiments of the devices or methods according to the different aspects of the invention can be combined in order to achieve advantageous light-emitting devices.